STRUGGLING TO GAIN WEIGHT!

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I'm out on a limb so I'm posting this with out feeling guilty of seeing the mass of members here to LOSE WEIGHT. So, here's my deal. I have been quite slim all my life I'm 5'9" so, it REALLY shows. After having my son, I finally got up to a weight of 130, which looked great! I maintained the weight for over 2 years with a combo of exercise and eating to maintain it. But, soon after had an abrupt period of sickness. I lost 20lbs!

Needless to say, I've taken care of my health issues. But now I am struggling to get my weight back up. I have always had an extremely fast metabolism and I consider my self to be quite athletic. Never had to restrict my diet. I eat all day, what ever, when ever, still.... No Progress.

HELP!